In known types of nuclear reactors the reactor core comprises a plurality of individually removable fuel assemblies arranged in an array capable of self sustained nuclear fission reaction. Each such removable fuel assembly typically comprises an array of elongated, cladded fuel elements or rods containing suitable fuel material, such as uranium oxide or plutonium oxide, or both, supported in spaced relation between upper and lower tie plates. A typical fuel assembly of this type is shown, for example, by B. A. Smith, et al in U.S. Pat. No. 3,689,358. An example of a fuel element or rod is shown in U.S. Pat. No. 3,378,458.
Additional information on nuclear power reactors may be found, for example, in "Nuclear Power Engineering", M. M. El-Wakil, McGraw-Hill Book Company, Inc., 1962.
To maintain operation, nuclear reactors typically are refueled on a periodic basis. That is, the reactor is periodically shut down and a portion of the fuel assemblies is removed from the core and is replaced with fresh fuel assemblies. The irradiated or spent fuel assemblies removed from the core must be stored for an indefinite, extended length of time for decay of radioactivity and heat liberation therefrom and until they can be removed for reprocessing or other disposition.
It is well known to store such fuel assemblies at a sufficient depth in a water-filled storage pool to provide radiation shielding, the water also acting as a coolant. In the past the fuel assemblies were widely spaced from one another in the pool to avoid the possibility of a critical array. The resulting low fuel assembly densities and storage capacities are found inadequate for current storage needs.
Various storage racks and storing devices have been proposed for reducing the substantial spacing between fuel assemblies in a storage pool. U.S. Pat. No. 4,010,375 discloses a rack for storage of spent nuclear fuel assemblies. The rack consists of an array of storage cells containing the fuel assemblies and interspersed between the cells are poison boxes consisting of water-filled enclosures having walls incorporating a neutron-absorbing material. The poison boxes function as neutron traps, and the storage cells and poison boxes are arranged in such a manner that each poison box is immediately adjacent at least one storage cell and adjacent storage cells are in contact on at least one side of each cell.
U.S. Pat. No. 3,859,533 describes a storing device for fuel assemblies comprising a supporting grid with a substantially square lattice and a plurality of tubes of substantially square cross section with each tube being adapted to contain one fuel element. Retaining devices are provided for holding the tubes in position with respect to the grid. The walls of each tube are coated with neutron absorbing material. Each of the retaining devices is arranged to support the walls of adjacent tubes in spaced relation to provide a gap between them. The gap contains a layer of neutron absorbing material.
Components for use in storage racks and devices have also been proposed. U.S. Pat. No. 4,006,362 discloses a generally tubular shroud of rectangular cross section having flat side walls formed by inner and outer tubes of rectangular cross section spaced apart to provide a substantially uniform space therebetween. An aluminum-boron carbide sandwich is positioned in and substantially fills the spaces at the four sides of the shroud. The inner and outer tubes and the sandwiches are interlocked by outwardly extending protrusions or dimples formed in the shroud walls.
German Patent Application No. 2,604,863, published for inspection on Sept. 23, 1976, describes a device for safe, and at the same time, space-saving storage of a plurality of fissionable masses, in which each mass is separated from the neighboring mass by means of a neutron-absorbing shield, an interstice filled with a moderating substance and a further neutron-absorbing shield. The device is characterized in that all shields surrounding one mass are connected with one another by means of spacers which define a predetermined minimum neutron moderating distance.
It has remained desirable to provide a storage rack for the storage of nuclear fuel assemblies that can be constructed inexpensively, is comprised of a minimum number of component parts, is easily fabricated and which maximizes the storage density.
It is therefore an object of the invention to provide a storage module for nuclear fuel assemblies which maximizes the number of assemblies that can be stored in a given space and minimizes the material used in construction.